In a magnetic tape recorder a capstan servo control controls the transport speed of a magnetic tape. The magnetic tape then engages with the shaft of a capstan motor. The speed of the shaft is measured by counting clock pulses produced between two successive speed pulses. A speed error is obtained by the subtraction of a set value from the actually counted number of revolutions. Such a servo arrangement is known from DE-PS-28 33 981.
The speed pulses are customarily produced by optically scanning a tacho disc which is rigidly connected to the shaft of the capstan motor. For achieving sufficient control accuracy, a line graduation applied to the tacho disc should have a very fine structure for obtaining a maximum number of speed pulses per revolution of the tacho disc. However, the number of equidistant lines may not be arbitrarily increased for reasons of typography, because errors in the line graduation also detrimentally affect the accuracy of control.